The present invention relates to the measurement of carrier frequencies by frequency counting, and more particularly to a time variant frequency correction technique for measuring carrier frequencies, which technique eliminates uncorrelated errors introduced by variations in a local oscillator.
A spectrum analyzer provides frequency spectrum information in the form of a graph having frequency and amplitude as the axes. The spectrum analyzer displays each frequency component of a signal as an individual vertical deflection. For a continuous wave (CW) signal, such as a sine wave, a single vertical deflection at the appropriate frequency of the sine wave is displayed. More complex signals will display other vertical deflections representing harmonic components, distortion products and the like.
Basically a spectrum analyzer is a tuned receiver coupled to a cathode ray tube display. By sweeping a local oscillator and varying the frequency span, virtually any portion of the frequency range of a particular instrument can be displayed. Also, with selectable intermediate frequency (IF) bandwidths and logarithmic and linear detection, individual signals close together in frequency can be resolved so that simultaneous display of signals at various amplitudes is achieved.
In the past techniques for measurement of carrier frequencies of CW signals have been limited to the direct methods of frequency counting. Recently modern spectrum analyzers have incorporated these traditional techniques to enhance the overall performance of and basic measurement capabilities of these types of instruments. In these applications additional errors are introduced by uncorrelated errors in the local oscillator systems due to periodic or random variations in local oscillator frequencies. As more and more items are counted to derive the final equation for input frequencies, the errors of these sources add and cause an additional error in the final result. Currently this counting of an intermediate frequency and of the local oscillator, the values of which are used by a microprocessor to compute an input frequency, is done sequentially. The errors accumulated due to variations in the local oscillator translate into an error in the value calculated for the input frequency equal to the error accumulated due to variations in the local oscillator during the measurement of the intermediate frequency plus the error accumulated during the measurement of the local oscillator frequency.
What is desired is a time variant frequency correction technique which removes all errors except those due to a reference frequency.